There will be an estimated 147,500 colorectal cancer cases diagnosed and 57,100 deaths from this disease in 2003. Therefore, the elucidation of preventable risk factors for colorectal neoplasia is an essential undertaking. It has long been hypothesized that diet may be associated with risk of colorectal cancer, and the search for the protective dietary components continues. Fiber has been studied intensively in the past with equivocal results. The Wheat Bran Fiber (WBF) and Polyp Prevention Trials (PPT) were two large, randomized, Phase Ill trials that failed to find a protective effect of either a cereal fiber supplement or a high-fiber, low-fat diet on colorectal adenoma recurrence respectively. In contrast, the results of two large studies recently indicated that fiber does confer protection from colorectal adenomas and cancer. Therefore, ambiguity with regard to the role of fiber remains. The hypothesis of the current work is that fiber may have more specific protective effects than previously investigated. These include that it may exert the greatest effect on advanced adenomas as compared to non-advanced, that the effect of fiber supplementation may be more pronounced in those who had consumed higher quantities of fiber prior to entry into the intervention trials, that the source of dietary fiber is an important consideration, and that there may be differential effects of fiber depending on gender and location in the colon. Pooling the study populations from the WBF and PPT trials provides an excellent opportunity to investigate these factors with data from 3209 study participants. The statistical power afforded by this large study size will enable further clarification of the association between fiber and colorectal neoplasia. If it is found that certain groups may receive greater benefit from fiber, then public health recommendations can be made which target these groups and prevent the recurrence of colorectal adenomas.